To promote reliability and prolong the operating life of a large steam turbine, it is imperative that excessive thermal stresses be avoided during all operating phases of the turbine. This includes loading and unloading the turbine with respect to a target load. Upon turbine startup, thermal stresses result from a mismatch between the temperature of the admitted steam and the turbine metal temperature. The degree of mismatch and the potential for excessive stress depend on recent operating history and on the point from which startup is begun, i.e., whether the turbine is involved in a hot start or a cold start. Once the turbine is started and producing load, however, steam flow is high enough that surface metal temperature closely follows steam temperature and overstressing can then be caused by rapid, uncontrolled changes in load.
Control of thermal stress is based primarily on analytical and statistical correlation between stress levels and expected rotor life. In the past, charts, graphs, and other control methods have been devised to guide the operator during the acceleration phase of the startup and to determine and control rates of change of metal temperature during the loading procedure. Various techniques have also been employed to speed up the loading process, including periods of heat soaking on "turning gear" to reduce the initial temperature mismatch. In addition, initial operation in the less efficient "full arc" steam admission mode is used to achieve uniform warming of the high pressure turbine inlet parts.
There have been a number of suggestions in the published prior art of methods to start and control steam turbines so that startup time can be minimized without inflicting damage on the turbine. However, these methods are usually predicated on ideal boiler conditions rarely existing in practice. Since turbine startups can take several hours, systems which reduce startup and loading and unloading times while allowing for fluctuations in steam temperature and pressure are of great value.
Sophisticated approaches to startup and loading control by means of continuously calculating rotor surface and bore stresses from speed and temperature measurements, and then loading to a maximum permissible stress are described in U.S. Pat. No. 3,446,224 to E. E. Zwicky, Jr. and in U.S. Pat. No. 3,561,216 to J. H. Moore, Jr., the disclosures of which are incorporated herein by reference thereto. Although these patents disclose methods and apparatus for achieving rapid startup and loading, faster results are desirable and can be expected through better thermal stress distribution among various parts of the different turbine sections relative to their design capabilities. Accordingly, it is among the objects of the present invention to provide an improved method and apparatus for controlling thermal stress on the component parts of a steam turbine while providing maximum loading and unloading rates during startup, shutdown, and other periods of load change.